retrogression of long waves. These rules are discussedin the following text.PROGRESSION OF LONG WAVES.Progression (eastward movement) of long waves isusually found in association with relatively short wavelengths and well defined major troughs and ridges. Atthe surface, there are usually only one or two prominentcyclones associated with each major trough aloft.Beneath the forward portion of each major ridge thereis usually a well developed surface anticyclone movingtoward the east or southeast. The 24-hour heightchanges at upper levels usually have a one-to-oneassociation with major troughs and ridges; that is,motion of maximum height fall and rise areas associatedwith major trough and ridge motion. The tracks of theheight change centers depend on the movement andchanges in intensity of the long waves.STATIONARY LONG WAVE PATTERNS.Once established, stationary long wave patterns usuallypersist for a number of days. The upper airflowassociated with the long wave pattern constitutes asteering pattern for the smaller scale disturbances (shortwaves). These short waves, with their associated heightchange patterns and weak surface systems, move alongin the flow of the large scale, long wave pattern. Shortwave troughs deepen as they move through the troughsof the long waves, and fill as they move through theridges of the long waves. The same changes in intensityoccur in sea level troughs or pressure centers that areassociated with minor troughs aloft. Partly as a resultof the presence of these smaller scale systems, thetroughs and ridges of the stationary long waves are oftenspread out and hard to locate exactly.RETROGRESSION OF LONG WAVES. Acontinuous retrogression of long wave troughs is a rareevent. The usual type of retrogression takes place in adiscontinuous manner; a major trough weakens,accelerates eastward, and becomes a minor trough,while a major wave trough forms to the west of theformer position of the old long wave trough. New majortroughs are generally formed by the deepening of minortroughs into deep, cold troughs.Retrogression is seldom a localized phenomenon,but appears to occur as a series of retrogressions inseveral long waves. Retrogression generally begins ina quasi-stationary long wave train when the stationarywavelength shows a significant decrease. This canhappen as a result of a decrease in zonal wind speed, orof a southward shift in the zonal westerlies. Somecharacteristics of retrogression are as follows:l Trajectories of 24-hour height change patterns at500-hPa deviate from the band of maximum wind.l New centers appear, or existing ones rapidlyincrease in intensity.. Rapid intensification of surface cyclones occursto the west of existing major trough positions.Location Of The Jet StreamThe AG2 TRAMAN, volume 1, discusses themigration of the jet stream both northward andsouthward. Some general considerations can be madeconcerning this migration and the movement of wavesin the troposphere:. In a northward migrating jet stream, a west windmaximum emerges from the tropics and graduallymoves through the lower midlatitudes. Anothermaximum, initially located in the upper midlatitudes,advances toward the Arctic Circle while weakening.Open progressive wave patterns with pronouncedamplitude and a decrease in the number of waves due tocutoff centers exist. The jet is well organized andtroughs extend into low latitudes.. As the jet progresses northward, the amplitude ofthe long waves decrease and the cutoff lows south of thewesterlies dissipate. By the time the jet reaches themidlatitudes, a classical high zonal index (AG2TRAMAN, volume 1) situation exists. Too, we haveweak, long waves of large wavelength and smallamplitude, slowly progressive or stationary. Fewextensions of troughs into the low latitudes are present,and in this situation, the jet stream is weak anddisorganized.. As the jet proceeds farther northward, there willoften be a sharp break of high zonal index with rapidlyincreasing wave amplitudes aloft. Long wavesretrograde. As the jet reaches the upper midlatitudes andinto the sub-Arctic region, it is still the dominant feature,while a new jet of the westerlies gradually begins toform in the subtropical regions. Long waves now beginto increase in number, and there is a reappearance oftroughs in the tropics. The cycle then begins again.With a southward migrating jet, the processes arereversed from that of the northward moving jet. Itshould be noted that shortwaves are associated with thejet maximum and move with about the same speed asthese jet maximums.2-3